Detection of brain activity is useful for medical diagnostics, imaging, neuroengineering, brain-computer interfacing, and a variety of other diagnostic and consumer-related applications. For example, cerebral blood flow ensures the delivery of oxygen and needed substrates to tissue, as well as removal of metabolic waste products. Thus, detection and quantification of cerebral blood flow is useful for diagnosis and management of any brain injury or disease associated with ischemia or inadequate vascular autoregulation.
As another example, there is an increasing interest in measuring event-related optical signals (also referred to as fast-optical signals). Such signals are caused by changes in optical scattering that occur when light propagating through active neural tissue (e.g., active brain tissue) is perturbed through a variety of mechanisms, including, but not limited to, cell swelling, cell volume change, cell displacement, changes in membrane potential, changes in membrane geometry, ion redistribution, birefringence changes, etc. Because event-related optical signals are associated with neuronal activity, rather than hemodynamic responses, they may be used to detect brain activity with relatively high temporal resolution,
Diffusive correlation spectroscopy (DCS), also referred to as diffusive wave spectroscopy (DWS), is a non-invasive optical procedure that has been shown to be effective in measuring some types of brain activity, such as cerebral blood flow. A conventional DCS system directs high coherence light (e.g., a laser) at a head of a subject. Some of the light propagates through the scalp and skull and into the brain where it is scattered by moving red blood cells in tissue vasculature before exiting the head. This dynamic scattering from moving cells causes the intensity of the light that exits the head to temporally fluctuate. To detect these temporal fluctuations, a conventional DCS system includes a photodetector and a correlator. The photodetector detects individual photons in the light that exits the head. The correlator keeps track of the arrival times of all photons detected by the photodetector and derives an intensity correlation function from temporal separations between the photons. This intensity correlation function is representative of the temporal fluctuations of the intensity of the light that exits the head, and is therefore also indicative of blood flow.
A conventional photodetector requires approximately one second to acquire enough signal for a meaningful measurement by a conventional DCS system. This is sufficient to detect changes in blood flow, which occur at relatively slow time scales (e.g., one second or more). However, conventional DCS systems do not operate fast enough to detect event-related optical signals caused, for example, by cellular activity, which occurs at a much faster rate than changes in blood flow.